Electric resistance apparatus.



No. 739,181. PATBNTED SEPT. 15, 1903.

R. HOPE-BLT;

ELECTRIC RBSISIAHGB APPARATUS. APB'LIGATIQK Hum JAIL as. 1902.

N0 MODEL.

WITNESSES INVENTOR m W W Q I g {fa "4 I ATTORNEYS "m: NoRms PETERS co. womumm WASHlNGYON. 1m:v

UNITED STATES Patented September 15, 1903.

ROBERT RoPEELT, 0 BERLIN, GERMANY.

ELECTRIC RESISTANCE APPARATUS.

SPECIFICATION forming part of Letters Patent No. 739,181, dated September 15, 1903.

Application filed January 28, 1902. 1 Serial No. 91,604. (No model.)

To all whom it may concern.-

Be it known that I, ROBERT HOPFELT, a subject of the German Emperor, residingin Berlin, in the German Empire, have invented certain new and useful Improvements in Electric Resistance Apparatus, of which the following is a full, clear, and exact description.

The attempt hasbeen made to use graphite resistances, which stand an extremely high temperature and can store up a greater amount of heat than any other kind, and when one uses graphite as a resistance material there is experienced the further advantage of having at disposal a material that is very cheap and is incombustible, so that such a resistance device is considerably more durable than the usual metallic ones. Such graphite resistances were formerly constructed like the metallic ones in that there were employed metallic contact-plates which were pushed into the graphite mass, eventually switching out the resistance by metallic contact. The 7 mass heretofore employed consisted of flake-graphite. When the knifecontacts cut into the pulverized mass, they form a furrow or groove, which when the knife is withdrawn must be refilled with graphite, else the contact between the knives and the graphite would beimperiled. Naturally the graphite mass will by this disturbance partly fly away, and for this reason the cutting out of the resistance in the same box that contains the graphite is impossible. It is therefore necessary to provide a currentinterrupter outside the box. Such resistances have in some cases proved satisfactory.

It has been discovered that pulverized graphite has the remarkable property that its resistance varies with the current After long experimenting I havefound that this quality of pulverized graphite thus used in a resistance deviceis exactly similar to the action of a coherer in a wireless-telegraphy apparatus and for convenience I will refer to this as a coherer action. Small sparks are formed between the individual carbon particles when the coherer action takes place, so that the graphite then participates only partially in the conduction of the current, the air being the chief agent therein. The change in the resistance of such a resistance device is considerable. A resistance of thirty-three ohms was in one instance reduced to only two ohms, the tensison being constant at one hundred and fifty volts. As soon as one smartly raps such a resistance apparatus the resistance is at once increased, despite the passage of the current. From this we can see that it is not the heating effect of the current but this coherer action that causes the change in- As, however, carbon particles cannot melt together their resistance changes of its own accord after the current is cut 0E. As soon as the current becomes constant the change in the resistance ceases, as the coherer action is principally dependent upon the magnitude or volume of the current and hardly upon the element of time. With constant current the resistance reaches after a few seconds a constant value, but decreases orincreases at once with increase or decrease of the current. The employment of such resistance-boxes as starting-rheostats for electric motors is therefore almost ideal, as all sudden changes are thereby absolutely'prevented. As metallic particles are easily sublimated, I employ for contact-plates solid carbon plates. The Sublimated metal would cover the carbon particles after awhile, and the resistance apparatus will, like a' metallic coherer, diminish in resisting efiectiveness, but will not increase again therein by shaking or jarring. It is then not advisable in the case of such resistance devices to bring the pulverized resistance mass, consisting of graphite or a similar substance, in contact with metal of any kind, for the graphite is easily brought to a glow and can under certain conditions attain such a high temperature that the conducting-plates when consisting of iron, copper, or similar metal would be melted. The

metal would be snblimated, and the so-called coherer action of the pulverized graphite would be prevented. To avoid this evil, the

pulverized carbon mass is, according to the carbon plates upon the metal plates themselves. Afterthebakingthecarbonplatesadhere closely to the metal. The metal plates can be provided with small projections, which hold the carbon plates, but which, however, must not project clear through the latter. The conduction'of the'current takes place through the solid carbon plates practically by means of the metal plates, so that the metal plates with carbon facings in combina-- solid carbon plates prevent the direct passage of the current between the powdered carbon mass and the metal plates.

In the accompanying drawings I have illus-' trated a resistance constructed as described. Figure'l is a side elevation. Fig. 2 is a longitudinal section; and Fig. 3 isacross-section on the line 3 3, Fig. 2.

-In the figures, 1) represents contact-plates in contact with the graphite mass f, which is confined in place between the contact-plates by the insulating rings orframes a, held between the plates b. Any suitable number of a the current entering through any such plate will flow through the graphite mass, then through the next contact-plate, and so on.

I claim as my invention- 1. An electric resistance device having frames, in combination with contact-plates arranged between the frames and covering 2. An electric resistance device having tion with a resistance mass of pulverized carbon material confined between andin contact with the carbon facings of the plates and adapted to give coherer action on a passage of current, substantially as described.

In testimony whereof I"have signed my name to this specification in the presence of two subscribing witnesses.

ROBERT HOPFELT.

Witnesses:

AUGUST SIEGFRIED DOCEN, PAUL STAAL. 

